My TR project

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Harry
Posts: 11
(4/5/04 11:55 pm)
 My TR project
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 OK, here is the thread I'll use for this amp.
I still have a long way to go, and I haven't ordered most of the parts yet. I have had a few questions so I thought I'd start the thread so I can get started asking.

This will be a Hoffman layout slightly modified AB763, Twin Reverb.
If I wanted to use an 8 Ohm load instead of 4 Ohm as well as reduce the power of the amp, would it be as simple as pulling 2 of the power tubes and running into an 8 Ohm load instead of a 4 Ohm load? Since I need to get 2 new speakers, I was thinking for the short term using a pair of EVs I have in hand, with the 4 tubes and replacing them with a pair of 16 ohm speakers when the time comes and just pull the 2 tubes. The idea that this could be done came from the many vibroclone projects using TR transformers.
 
EL34  
I got your parts right here.
Posts: 2269
(4/6/04 4:09 pm)

  Re: My TR project
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 Why would you pull two tubes?

Why make a twin if you only want two tubes?

Make a 2-6L6 amp and use the 018343 output which has 2,4 and 8 ohm speaker taps.

An AB763 circuit can be many different models. Same circuit, different tranny's and tubes.

Edited by: EL34   at: 4/6/04 4:10 pm
 
Harry
Posts: 12
(4/6/04 10:11 pm)
 My TR project
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 I already have all the existing transformers. I don't want to throw more money at it. However IF I had to replace the speaker transformer I would certainly go with something designed for what I would prefer which would likely be a more Super Reverb sized amp.

So the question remains, would the ratio be correct for 2 tubes with an 8 Ohm load, for a transformer designed to be used with 4 tubes and a 4 Ohm load?

If I want to use the amp all out I could still plug 2 tubes back in and use an extension cabinet.

This is all for the sake of theory. When it's finished I may just sell it to buy a DR anyway.

Thanks for the advice.
 
66barry
Junior tube assistant
Posts: 3
(4/7/04 1:07 am)
 Re: My TR project
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 Het Harry, Yes when pulling the two outer tubes or two inner tubes from your amp with a normal output of 4 ohms the amp needs to see a 8 ohm load.  
 
GroundhogKen
Forum Moderator
Posts: 2340
(4/7/04 7:18 am)
 Re: My TR project
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 For the sake of getting up and running:

If you have a Twin transformer (2000 ohm : 4 Ohm ) and two parallel 8 ohm speakers which are capable of handling the four 6L6s, then you could pull one pair of tubes and disconnect one speaker.

The single 8 ohm speaker will reflect the proper 4000 ohms onto the remaining pair of tubes, and since two speakers handled the output of four tubes, one speaker will handle the output of two tubes.

If you don't mind running a single speaker, then this saves you having to buy a pair of 16 ohm speakers in the future.


Ken
 
Harry
Posts: 13
(4/7/04 10:43 am)
 Re: My TR project
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 Thanks for the info. I really don't know how all this fits together yet, but I am trying to learn.
So would a single 6L6 need to drive 8K in class A?
 
GroundhogKen
Forum Moderator
Posts: 2341
(4/7/04 3:30 pm)
 Re: My TR project
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 No, unfortunately that would be too simple. Single-ended designs are a little different than push-pull.


Ken
 
Harry
Posts: 14
(4/22/04 1:53 am)
 Re: My TR project
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 Could someone explain the curcuit from the phase invertor tube to the output transformer and compare the original to the hoffman modded version?
 
GroundhogKen
Forum Moderator
Posts: 2350
(4/22/04 7:28 am)
 Re: My TR project
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 The Fender AB763 and the Hoffman AB763 are identical with one exception. The Hoffman circuit uses bias vary tremolo instead of the photoresistor ( roach ) type.

Most people prefer the bias vary tremolo. It can be found in amps such as Brown Vibroverbs and Blackface Princeton Reverbs. All the other Blackface amps used the photoresistor type.

Neither type is as cool as the harmonic vibrato that was used in brown Concerts, Supers, Pros, Twins and Showmans, but to implement that circuit takes a couple extra tubes.

Ken
 
johnt1
Junior tube assistant
Posts: 16
(4/22/04 8:29 am)
 tr project
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 Ken, how would sound differ between the bf Princeton and 5g13 circuit. (vibrato section) I just completed my revibe and I'm not sure what the vibrato section of the 5g13 should sound like.Is it simalar to the units you have described. I still have a few kinks to iron out.
 
GroundhogKen
Forum Moderator
Posts: 2351
(4/22/04 9:34 am)
 Re: tr project
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 The the brown 2 1/2 tube vibrato has a cosmic underwater bubbly sound.

The bias vary tremolo is nice, wavy and smooth, but it tends to lose its potency at higher volumes.

the blackface photoresistor tremolo sounds like child spinning the volume control on and off and is sometimes accompanied by an annoying ticking sound.

How's that for a precise description?


Ken
 
johnt1
Junior tube assistant
Posts: 17
(4/22/04 9:55 am)
 Re: tr project
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 Ken thank you, oddly enough that dose describe what I am hearing. My revibe build must not be that far off. I just need to cure the motor boat on on the vibrato section and On will have it complete.
Thanks John T.
 
Harry
Posts: 15
(4/24/04 3:19 am)
 Re: My TR project Anyone care to answer this one?
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 Still looking for an answer.

Thanks.
 
GroundhogKen
Forum Moderator
Posts: 2352
(4/24/04 5:36 pm)
 Re: My TR project Anyone care to answer this one?
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 Harry, if you are looking for a technical explanation of a long tail phase inverter and a push pull output stage, then you should check out these two links:


www.aikenamps.com/LongTailPair.htm

www.aikenamps.com/SingleEnded.htm


In fact, Randall Aiken's Tech Info papers are probably the best online resource concerning vintage guitar amps

www.aikenamps.com



Ken
 
Harry
Posts: 16
(4/25/04 1:17 am)
 Re: My TR project Thanks
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 thanks Ken
 
 
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